1. Field of the Invention
The present application relates to a wireless battery charger for mobile or portable devices (e.g., cellular telephones). In this context, “wireless” refers to the fact that the charger charges the battery of a portable device through a mechanism that does not involve connecting the portable device to the charger by a cable. In particular, the present application relates to a universal wireless battery charger that does not have the disadvantages of a battery charger that is based on inductive coupling.
2. Discussion of the Related Art
There are two main categories of wireless battery chargers for portable devices. One category transfers energy by providing direct contact between the portable device and the charger or the charging base station (“direct contact charger”). The second category relates to using an electromagnetic field to transfer energy between the portable device and the wireless charger. A wireless battery charger that transfers energy through inductive coupling (“inductive charger”) typically includes a first induction coil that creates an alternating electromagnetic field from the inductive charger. The alternating electromagnetic field from the inductive charger then induces an electrical current in a second induction coil in a properly positioned the portable device. This inducted electrical current then charges the battery of the portable device. The two induction coils in proximity together form, essentially, an electrical transformer.
Charging by inductive coupling suffers from many disadvantages. For example, relative to charging by direct contact, an inductive charger typically has low efficiency and experiences resistive heating. As the lack of efficiency manifests itself in energy loss in the form of heat, an inductive charger gets quite warm during charging operations. The resulting elevated temperature causes undue stress on the battery being charged. Consequently, the batteries in portable devices that are charged by such a charger do not last as long as those regularly charged by a plug-in charger. There are two additional aspects of an inductive charger that should be noted. First, the heat build-up occurs only during charging operations. Second, the efficiency that is achieved also depends heavily on the relative positions of the two coupled coils. Implementations using a lower frequency or using an older drive technology may charge more slowly and generates heat within most portable electronic devices. Further, inductive charging requires specific electronic circuits and coils in both the inductive charger and the mobile device being charged, thereby resulting in both increased complexity and increased cost of manufacturing such devices. See, for example, U.S. Pat. No. 6,972,543, entitled “Series Resonant Inductive Charging Circuit,” issued Dec. 6, 2005. The public is also concerned about the alternating electromagnetic fields, which typically operates at a radio frequency within the range of 80-300 kHz. Even at 5 watts, such electromagnetic fields may still cause significant health concerns to human beings nearby. Some stations transmit at 915 MHz, which is the frequency often used to heat food in microwave ovens.
A wireless direct contact charger transfers energy without the disadvantages of an inductive charger. One way of implementing a wireless direct contact charger is to provide “point-to-point electrodes” i.e., providing electrodes in a base station of the charger that couple with corresponding electrodes in the portable device being charged, e.g., a home wireless telephone. Such a charging arrangement requires that the electrodes on the portable device be completely aligned to the corresponding electrodes in the base station, and be oriented in the correct polarities. Another way of implementing a direct contact charger is to provide “multiple point electrodes,” such as providing electrodes in the form of strips. One example of a multi-point direct contact charger is a charger marketed under the trade name “Wildcharge System.” In that system, the electrodes are provided as numerous parallel strips of alternating polarities on a charging surface of the housing of the direct contact charger. A portable device to be charged is required to have multiple electrodes formed along the circumference of a small circle. The electrodes are usually located at the center of mass of the portable device, so that when the portable device is placed on the charging surface of the charger, the multiple electrodes on the portable device support the portable device on the charging surface without tilting. If device is tilted, electrical contact with the charger would be lost. If the portable device is placed in an incorrect position, charging operations may fail (e.g., when two of the electrodes on the portable device sit in between adjacent electrode strips).